Tb3+-Triactivated Mg2Y8

ARTICLE pubs.acs.org/JPCC

Color Tuning Luminescence of Ce3+/Mn2+/Tb3+-Triactivated Mg2Y8(SiO4)6O2 via Energy Transfer: Potential Single-Phase White-Light-Emitting Phosphors Guogang Li,†,‡ Dongling Geng,†,‡ Mengmeng Shang,†,‡ Yang Zhang,†,‡ Chong Peng,†,‡ Ziyong Cheng,† and Jun Lin*,† †

State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China ‡ Graduate School of the Chinese Academy of Sciences, Beijing 100049, P. R. China

bS Supporting Information ABSTRACT: Ce3+, Mn2+, and Tb3+-activated Mg2Y8(SiO4)6O2 (MYS) oxyapatite phosphors have been prepared via solid state reaction process. The Ce3+ emission at different lattice sites in MYS host has been identified and discussed. Under UV excitation, there exist dual energy transfers (ET), that is, Ce3+ f Mn2+ and Ce3+ f Tb3+ in the MYS: Ce3+/Mn2+/Tb3+ system. The energy transfer from Ce3+ to Mn2+ in MYS: Ce3+/Mn2+ phosphors has been demonstrated to be a resonant type via a dipole
quadrupole mechanism, and the critical distance (RC) calculated by quenching concentration method and spectral overlap method are 10.5 and 9.7 Å, respectively. The emitting colors of MYS: Ce3+/Mn2+/Tb3+ samples can be adjusted from blue to orange-red via ET of Ce3+ f Mn2+ and from blue to green via ET of Ce3+ f Tb3+, respectively. More importantly, a widerange-tunable white light emission with high quantum yields (37
47%) were obtained by precise control of the contents of Ce3+, Mn2+, and Tb3+ ions. On the other hand, the CL properties of MYS: Ce3+/Mn2+/Tb3+ phosphors have been investigated in detail. The results indicate that the as-prepared MYS: Ce3+/Mn2+/Tb3+ phosphors have good CL intensity and CIE coordinate stability with a color-tunable emission crossing the whole visible light region under low-voltage electron beam excitation. In conclusion, the white light with varied hues has been obtained in Ce3+, Mn2+ and Tb3+-activated MYS phosphors by utilizing the principle of energy transfer and properly designed activator contents as well as the select of excitation wavelength under UV and low-voltage electron beam excitation.

1. INTRODUCTION Lanthanide-ion-doped phosphors are efficient luminescent materials and irreplaceable components of light-emitting devices, such as cathode ray tubes (CRTs), plasma display panels (PDPs), field emission displays (FEDs), and white light emitting diodes (WLEDs). In recent years, much attention has been paid to the generation of white light sources for a variety of applications, including solid-state multicolor three-dimensional displays, illumination, back light, and so on.1
3 Moreover, many efforts have been made to obtain white light emission with a high color rendering index using phosphors to convert ultraviolet (UV), blue, or infrared light into a combination of RGB.1,2,4 However, in the three-converter (RGB) system, the manufacture cost is high and the blue emission efficiency is poor because of the strong reabsorption of blue light by the red, green-emitting phosphors. Therefore, to circumvent these disadvantages, the investigation of efficient, durable, and single-phase white-light-emitting phosphors with RGB components through energy transfer between activators such as Ce3+fMn2+ 5,6 and Ce3+fTb3+ 7,8 are drawing much attention. So far, the Ce3+ f Mn2+ energy transfer r 2011 American Chemical Society

mechanism has been investigated in many inorganic hosts, such as fluorides,9 phosphates,10 and borates,11 while the Ce3+ f Tb3+ energy transfer has also been extensively reported.7,8 In these systems, there is a common feature that Ce3+ and Mn2+/Tb3+ ions simultaneously substitute for one or two host lattice sites and the Ce3+ ions serve as effective sensitizer ions not only help Mn2+ and Tb3+ ions to emit efficiently but also tune their emission colors from blue to orange/red and from blue to green, respectively. Furthermore, a single-phase white-light-emitting phosphor utilizing energy transfer can avoid the reabsorption for blue or UV light by the red-/green-emitting phosphors and the mixing of RGB phosphors. Consequently, it can enhance the luminescence efficiency and color reproducibility of the white light source, and reduce manufacturing costs. In addition, Zhang et al.12 has reported a novel strategy to obtain white light emission in a single-phase Ce3+ and Mn2+-activated Ca5(PO4)3F host through Received: May 24, 2011 Revised: September 15, 2011 Published: October 10, 2011 21882

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The Journal of Physical Chemistry C the modulation of UV excitation wavelength. Evans and Liu et al.13 also confirmed a color-tunable emission can be obtained by the change of excitation wavelength. On the other hand, the field emission displays (FEDs) have been considered as one of the most promising next generation flat panel displays due to their potential to provide displays with thin panels, self-emission, wide viewing, quick response time, high brightness, high contrast ratio, lightweight, and low power consumption.14 For FEDs, phosphors must be operated at significantly lower excitation voltages (e5 kV) and higher current densities (10
100 μA/cm2) than CRTs.15,16 Thus, the phosphors for FEDs are required to have a higher efficiency at low voltages, higher resistance to current saturation, longer service time and equal or better chromaticity than CRT phosphors. To realize the full-color FEDs, it is necessary to develop some novel phosphors with good stability and high efficiency as well as color-tunable emission.17
19 Single-phased phosphors with multicolor-tunable emission via energy transfer are good candidates for this application in FEDs. Generally, the availability of high-quality phosphors operating under UV and lowvoltage electron beam excitation is of prime importance for better performance of UV-LEDs and FEDs. In the case of the WLEDs and FEDs, inorganic oxide-based phosphors would be the best candidates in terms of chemical stability, environmentally friend and luminescence efficiency.17
19 Moreover, rare-earth and transitional-metal-doped, oxide-based phosphors for LEDs and FEDs have been of great interest because of their excellent light output, color-rendering properties, and superior stability under UV and electron bombardment.17
19 It is well-known that the compounds with oxyapatite structure have been effective host lattices for luminescent materials due to their applications in solid-state lighting and display industry.20
23 Moreover, they have excellent chemistry stability. Among the many synthetic oxyapatites, the ternary rare-earthmetal silicate Mg2Y8(SiO4)6O2 is the efficient host lattice for the luminescence of various RE3+ (Ce3+, Tb3+, Dy3+, Sm3+, and Eu3+) ions and mercury-like ions.24 This oxyapatite host lattice consists of two cationic sites, that is, the 9-fold coordinated 4f sites with C3 point symmetry and 7-fold coordinated 6h sites with Cs point symmetry.23 Both sites are suitable and easily accommodate a great variety of rare earth- and transitional-metal ions. To the best of our knowledge, up to now, there are only several reports about RE3+ (Ce3+, Sm3+, Eu3+, Tb3+, and Dy3+)-doped MYS phosphors and rare reports have been found on the detailed photoluminescence (PL) properties of Mn2+ and the sensitization effect of Ce3+ on Mn2+ in the MYS host lattice.20
23 The dual energy transfer induced Ce3+, Mn2+, Tb3+-triactivated MYS phosphors has not been investigated yet. In addition, the cathodoluminescence (CL) properties of Ce3+, Mn2+-coactivated and Ce3+, Mn2+, Tb3+-triactivated MYS phosphors have not been reported so far. Herein, we reported the synthesis of MYS: Ce3+/ Mn2+/Tb3+ phosphors using conventional solid state reaction. The Ce3+ emission at different sites in MYS host was discussed. The energy transfer properties from Ce3+ to Mn2+ and from Ce3+ to Tb3+ were investigated and the critical distance (RC) of Ce3+ f Mn2+ was calculated. At the same time, the luminescence properties of MYS: Ce3+/Mn2+ phosphors by the modulation of excitation wavelength were investigated. More importantly, a color-tunable luminescence including white light in the MYS: Ce3+/Mn2+/Tb3+ phosphors was obtained by varying the relative doping concentrations of Ce3+, Mn2+ and Tb3+. Finally, the CL properties of MYS: Ce3+/Mn2+/Tb3+ phosphors were also investigated in detail.

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Figure 1. XRD patterns of (a) MYS, (b) MYS: 1 Ce3+, (c) MYS: 1 Ce3+, 15 Mn2+, (d) MYS: 1 Ce3+, 8 Tb3+ and (e) MYS: 1 Ce3+, 3 Mn2+, 4 Tb3+. The standard data for MYS (JCPDS No.20
1410) is shown as reference.

2. EXPERIMENTAL SECTION 2.1. Preparation. A series of polycrystalline Mg2Y8(SiO4)6O2: x mol % Ce3+, y mol % Mn2+, z mol % Tb3+ powder samples were prepared by conventional high temperature solid state reaction process. The expression of Mg2Y8(SiO4)6O2: x mol % Ce3+, y mol % Mn2+, z mol % Tb3+, in the following sections are abbreviated as MYS: x Ce3+, y Mn2+, z Tb3+, for example, the Mg2Y8(SiO4)6O2: 1 mol % Ce3+, 3 mol % Mn2+, 1 mol % Tb3+ are denoted as MYS: 1 Ce3+, 3 Mn2+, 2 Tb3+. The Ce3+/Tb3+ and Mn2+ ions substitute of Y3+ and Mg2+ in MYS host, respectively, and x, y and z are all mol %. The doping concentrations of Ce3+, Mn2+ and Tb3+ were chosen as 0.1
10 mol % of Y3+, 1
50 mol % of Mg2+ and 1
10 mol % of Y3+ in Mg2Y8(SiO4)6O2, respectively. Typically, stoichiometric amounts of MgO (Aldrich, 99.99%), SiO2 (Aldrich, 99.99%), Y2O3, CeO2, Tb4O7 (all 99.99%, Shin-Etsu Chemical Co. Ltd., Tokyo, Japan), and MnCO3 (Aldrich, 99.99%) were thoroughly mixed in an agate mortar for 1 h with an appropriate amount of ethanol and then dried at 90 °C for 2 h. The powder mixtures were sintered at 1350 °C for 2 h in a reducing atmosphere of H2 (5%) and N2 (95%) to produce the final samples. 2.2. Characterization. The X-ray diffraction (XRD) patterns were performed on a D8 Focus diffractometer at a scanning rate of 10° min
1 in the 2θ range from 15° to 70° with graphitemonochromatized Cu Kα radiation (λ = 0.15405 nm). The photoluminescence (PL) measurements were recorded with a Hitachi F-7000 spectrophotometer equipped with a 150 W xenon lamp as the excitation source. The cathodoluminescence (CL) measurements were conducted in an ultrahigh-vacuum chamber ( RC and energy transfer from Ce3+ to Mn2+ dominates when RCe
Mn < RC. This value is much longer than 4 Å, indicative of little possibility of energy transfer via the exchange interaction mechanism. Thus, the electric multipolar interaction can take place for energy transfer between the Ce3+ and Mn2+ ions. On the basis of Dexter’s energy-transfer expressions of multipolar interaction and Reisfeld’s approximation, the following relation can

Figure 5. Energy transfer efficiency from Ce3+ to Mn2+ in MYS: 1 Ce3+, y Mn2+ (y = 0, 1, 3, ..., 30 mol %) samples under different UV excitation (λex = 292, 324 nm).

be given:10,38,39 η0 µ C n=3 η

ð4Þ

where η0 and η are the luminescence quantum efficiency of Ce3+ in the absence and presence of Mn2+; C is the sum of the content of Ce3+ and Mn2+; n = 6, 8, and 10 corresponding to dipole
dipole, dipole
quadrupole, and quadrupole
quadrupole interactions, respectively. The value η0/η is approximately calculated by the ratio of related luminescence intensities as5,32 IS0 µ Cn=3 IS

ð5Þ

where IS0 is the intrinsic luminescence intensity of Ce3+, and IS is the luminescence intensity of Ce3+ in the presence of the Mn2+. The IS0/IS
Cn/3 plots are shown in Supporting Information Figure S3, and the relationships are observed when n = 6, 8, and 10. A linear relationship was only observed when n = 8. This clearly indicates that the energy-transfer mechanism from the Ce3+ to Mn2+ ions is a dipole
quadrupole type interaction. Therefore, the electric dipole
quadrupole interaction predominates in the energy-transfer mechanism from the Ce3+ and Mn2+ ions in MYS host. Considering the dipole
quadrupole interaction, the critical distance from sensitizer to acceptor can also be calculated by the spectral overlap method, as expressed as follows10,41 Z FS ðEÞFA ðEÞ dE RC 8 ¼ 3:024  1012 λS 2 fq ð6Þ E4 where fq (10
10) is the oscillator strength of the involved absorption transition of the acceptor (Mn2+), λS (in Å) is the wavelength position of the sensitizer’s emission, E is the energy R involved in the transfer (in eV), and FS (E) FA (E) dE/E4 represents the spectral overlap between the normalized shapes of the Ce3+ emission FS(E) and the Mn2+ excitation FA(E), and in our case it is calculated to be about 0.01468 eV
4. Using the eq 6, the critical distance RC was estimated to be 9.7 Å. This result is basically in agreement with that obtained using the concentration quenching method, which further reveals that the mechanism of energy transfer from the Ce3+ to Mn2+ ions is mainly due to a dipole
quadrupole interaction. It is well-known that rare earth ions have been playing an important role in modern lighting and display fields because of the abundant emission colors based on their 4f f 4f or 5d f 4f transitions.29 Generally, the Tb3+ ion are used as an activator in 21887

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Figure 6. Typical PLE and PL spectra of (a) MYS: 1 Ce3+, (b) MYS: 3 Tb3+, (c) MYS: 1 Ce3+, 8 Tb3+ samples. (d) The PL spectra of MYS: 1 Ce3+, z Tb3+ (z = 1, 5, 8 mol %) samples as a function of Ce3+-doping concentration (z, mol %).

green phosphors, whose emission is mainly due to transitions of 5D3 f 7FJ in the blue region and 5D4 f 7FJ in the green region (J = 6, 5, 4, 3, 2) depending on its doping concentration.7,8 Figure 6b shows the PLE and PL spectra of MYS: 3 Tb3+ sample. The excitation spectrum of MYS: 3 Tb3+ shows a broad band from 220 to 290 nm with a maximum at 237 nm and some weak transitions from 300 to 500 nm. The former is related to the 4f8
4f75d transition of Tb3+, and the latter is due to its intra-(4f) transitions. Under 237 nm UV radiation excitation, the asprepared MYS: 3 Tb3+ gives green emission, and the obtained emission spectrum consists of the f-f transition lines within the Tb3+ 4f8 electron configuration, that is, 5D3 f 7F6 (380 nm), 5 D3 f 7F5 (419 nm), 5D3 f 7F4 (438 nm), 5D3 f 7F3 (459 nm), 5 D4 f 7F6 (488 nm), 5D4 f 7F5 (544 nm), 5D4 f 7F4 (583 nm), and 5D4 f 7F3 (620 nm), respectively. The Ce3+ ion is a wellknown sensitizer for trivalent rare earth ion luminescence, and the sensitizing effects depend strongly on the host lattices into which these ions are introduced. The Tb3+ emission in Mg2Y8(SiO4)6O2 can be sensitized by Ce3+, which is because the emission band of Ce3+ in the range 350
550 nm overlaps with the excitation peaks of Tb3+ at 350
500 nm, as shown in the Figure 6a and 6c. A typical excitation and emission spectrum for a sample with composition MYS: 1 Ce3+, 8 Tb3+ is shown in Figure 6c. The excitation spectrum monitoring with Tb3+5D4 f 7 F5 emission (544 nm) clearly shows two bands: 220
290 nm centered at 237 nm and 290
390 nm with centers at 324 and 365 nm. The first band is due to absorption of Tb3+ 4f8
4f75d transition and the latter is caused by Ce3+ 4f1
5d1 transition. The presence of the Ce3+ excitation band suggests energy transfer from Ce3+ to Tb3+. In addition, upon excitation into the Ce3+ absorption band, the emission spectrum of MYS: 1 Ce3+, z Tb3+ (z = 1, 5, 8 mol %) samples (Figure 6d) not only shows both Ce3+ and Tb3+ emission but also the emission intensity of Ce3+ decreases with the increase of Tb3+ concentration (z, mol %), which further confirm the existence of energy transfer from Ce3+ to Tb3+. In view of the Ce3+fTb3+ energy transfer in MYS, the emission colors of MYS: 1 Ce3+, z Tb3+ (z = 0, 1, 5, 8 mol %) phosphors can be tuned from blue to green by changing the

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doping concentration of Tb 3+ ion. The quantum yields (48
76%) and CIE color coordinates of MYS: 1 Ce3+, z Tb3+ (z = 0, 1, 2, ..., 8 mol %) phosphors under 292 nm UV excitation are listed in Supporting Information Table S3. It is worth pointing out that the emission wavelength of Ce3+ shift from 425 nm (in MYS:1 Ce3+) to 408 nm with the codoping of Tb3+ (in MYS:1 Ce3+, z Tb3+). That is because that the f-d transition of Ce3+ ions is strongly related to the crystal structure and coordinated environment. The doping of Tb3+ ions in the MYS host will result in the slightly expand of crystal lattice, which decreases the crystal field strength and then make the Ce3+ emission have a blue shift. In addition, the preparation conditions and measure equipment also have an impact on the blue shift of Ce3+ emission. The energy transfer from sensitizer to activator is a feasible route to realize color-tunable emission, and a white light emission can be obtained through mixing the tricolor (RGB) light sources at a suitable ratio. In our case, the MYS: Ce3+ emits bright blue light because of the 5d f 4f transition of Ce3+, and the efficient energy transfer of Ce3+ f Mn2+ is also validated in Ce3+/Mn2+coactivated MYS phosphors. Thus, it is reasonable to predict that the color-tunable emissions from blue light to orange-red light in MYS: Ce3+, Mn2+ systems can be obtained. In addition, a colortunable emission from blue to green also can be realized in MYS: Ce3+, Tb3+ samples through the Ce3+ f Tb3+ energy transfer. Thus, it is possible to produce white emission due to the simultaneous appearance of RGB light by appropriately adjusting of the doping concentration of Mn2+ and Tb3+ when fixing the Ce3+ (sensitizer) concentration in the present MYS host. Our experiments have confirmed the above situation. However, the Ce3+ f Mn2+ and Ce3+ f Tb3+ energy transfer processes in MYS host are complicated process due to the slightly change of crystal structure and the interaction of Ce3+, Mn2+, and Tb3+ ions. Therefore, we neglect the interaction, concentration quench and energy transfer here. A series of MYS: 1 Ce3+, y Mn2+, z Tb3+ samples with different Mn2+ and Tb3+ concentrations (y = 0, 1, 3, ..., 50 mol %, z = 0, 1, 2, ..., 8 mol %) have been synthesized. The quantum yields (QYs) and CIE chromaticity coordinates (x, y) of the MYS: Ce3+, Mn2+, Tb3+ samples under UV excitation (λex = 292, 324 nm) are summarized in Supporting Information Table S3. As shown in Figure 4a, it can be observed that the emission spectra of MYS: 1 Ce3+, y Mn2+ samples are composed of blue emission of Ce3+ and orange-red emission of Mn2+ (4T1(4G) f 6A1(6S)), and the blue emission gradually decrease and orange-red emission gradually increase with the increase of Mn2+ concentration. Therefore, the emitting colors of MYS: 1 Ce3+, y Mn2+ samples are tuned from blue to orange-red by changing the Mn2+ concentrations, as suggested by the CIE coordinates in the arrowed line a of Figure 8. The quantum yields of MYS: 1 Ce3+, y Mn2+ (y = 1, 3, 5, ..., 50 mol %) samples are measured to be 21
54% for λex = 292 nm and 27
56% for λex = 324 nm, respectively. On the other hand, when codoping Ce3+ and Tb3+ into MYS host, the PL spectra of MYS:1 Ce3+, z Tb3+ samples consist of 5d f 4f transition of Ce3+ and 5D4 f 7FJ (J = 6
2) transitions of Tb3+, and the emitting colors of MYS:1 Ce3+, z Tb3+ samples can be tuned from blue to green due to the Ce3+fTb3+ energy transfer (Figure 6). These results can also be confirmed by their CIE chromaticity coordinates shown in Supporting Information Table S3 and arrowed line b of Figure 8. In general, it is clearly seen from Figure 8 that the CIE coordinates of MYS: 1 Ce3+, y Mn2+ (y = 0, 1, 3, ..., 50 mol %) and MYS: 1 Ce3+, z Tb3+ (z = 0, 1, 2, ..., 8 mol %) samples under 21888

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Figure 7. PL spectra of MYS: 1 Ce3+, y Mn2+, z Tb3+ samples ( y = 3, 5 mol %; z = 1, 2, 4 mol %) under different wavelengths UV excitation: (a) 292 nm and (b) 324 nm.

292 nm UV excitation move from blue region to orange-red region (arrowed line a) and from blue region to green region (arrowed line b) with the increase of y and z values, respectively. More importantly, a wide-range-tunable white emission can be obtained by precisely control the contents of Mn2+ and Tb3+ ions, as shown in Points 1
8. When fixing the y at 3 mol %, the CIE coordinate positions of MYS: 1 Ce3+, y Mn2+, z Tb3+ samples moves from point 1 to point 2, 3, 4 with the increase of z values from 1 to 2, 3, 4 mol %, namely, the increase of green component. The same situation is held for MYS: 1 Ce3+, 5 Mn2+, z Tb3+ (z = 1, 2, 3, 4 mol %) samples, for example, the point 5, 6, 7, 8 in Figure 8. When fixing the y at 2 mol %, the CIE coordinate positions of MYS: 1 Ce3+, 5 Mn2+, z Tb3+ samples moves from point 2 to point 6 with the increase of y values from 3 to 5 mol %, namely, the increase of red component. The similar results are suitable to point 1 f 5, point 3 f 7, and point 4 f 8. The above results indeed indicate that a tunable white emission can be obtained by precisely controlling the Ce3+, Mn2+, and Tb3+ concentrations in the MYS host. In addition to the energy transfer, changing excitation sources is another route to realize the color-tunable emission in a single phase host. In our study, we found that the blue emission parts and relative intensity of RGB emission can be tuned by the variation of excitation UV light. The former is attributed to the different Ce3+ emission at different lattice site, while the latter is due to the slight difference of energy transfer efficiency under different UV excitation. Figure 7 (black lines) gives the PL spectra of the representative MYS: 1 Ce3+, 3 Mn2+, 1 Tb3+ sample under different UV excitation. The blue emission at 425 nm (λex = 292 nm) and 456 nm (λex = 324 nm) can be assigned to Ce3+ (4f) and Ce3+ (6h) emission, respectively.24 Therefore, when changing the UV excitation wavelength from 292 to 324 nm, the blue emission has a red shift with the emission peak from 425 to 456 nm, and the relative PL intensity of green and red to blue emission increases, which results in the change of emitting colors such as the point 1 (0.315, 0.272) and point 9 (0.321, 0.303) shown in Figure 8. Generally, a wide-rangetunable white light emission from cool to warm white emission with high quantum yields (37
47%) were obtained in MYS: Ce3+, Mn2+, Tb3+ samples by the dual energy transfer of Ce3+fMn2+ and Ce3+ f Tb3+, as well as the use of different UV sources.

Figure 8. CIE chromaticity diagram for MYS: 1 Ce3+, y Mn2+, z Tb3+ (y, z mol %) samples under 292 nm UV excitation: (1) y = 3, z = 1; (2) y = 3, z = 2; (3) y = 3, z = 3; (4) y = 3, z = 4; (5) y = 5, z = 1; (6) y = 5, z = 2; (7) y = 5, z = 3; (8) y = 5, z = 4 and under 324 nm UV excitation: (9) y = 3, z = 1. a and b represents the variation of CIE coordinates of MYS: 1 Ce3+, y Mn2+ (y = 0, 1, 3, ..., 50 mol %) and MYS: 1 Ce3+, z Tb3+ (z = 0, 1, 2, ..., 8 mol %) samples, respectively.

3.3. Cathodoluminescence Properties. To explore the potential of the as-synthesized MYS: Ce3+, Mn2+, Tb3+ samples to be used as CL materials, their CL properties have been investigated in detail. Figure 9 show the CL spectra of MYS: x Ce3+, y Mn2+, z Tb3+ samples at different Ce3+, Mn2+, and Tb3+ concentrations (x = 0, 5 mol %, y = 0, 1, 3, ..., 18 mol %, z = 1, 2, 4, ..., 8 mol %), whose profiles are similar to their PL spectra. The insets 1
13 show the digital cathodoluminescence photographs of MYS: x Ce3+, y Mn2+, z Tb3+ samples. The CIE chromaticity coordinates of the MYS: x Ce3+, y Mn2+, z Tb3+ under low voltage electron beam (Accelerating voltage = 3.0 kV; Filament current = 90 mA) are summarized in Table 1. Under the lowvoltage electron beam excitation, pure MYS: 1 Ce3+ sample gives bright blue emission (Figure 9d, inset 1), whose emission spectrum consists of a broad band (370
500 nm) centered at 410 nm because of the 5d1 f 4f1 transition of Ce3+. The blue emission of MYS: 1 Ce3+ can be determined by its CIE coordinate (0.173, 0.087) calculated through its CL spectrum. The CL emission spectrum of MYS: 1 Ce3+ has a slight difference than that of PL spectrum, which could result from different excitation source. For pure Mn2+-activated MYS sample, the CL spectrum of MYS: 18 Mn2+ shows a broad band emission from 520 to 650 nm with the maximum at 579 nm due to the 4T1 f 6S1 transition of Mn2+. Its CIE coordinate is calculated to be (0.489, 0.351), which locates in the orange-red region, and its CL photograph is shown as inset 6 in Figure 9d. The difference in the maximum emission of Mn2+ between its CL and PL spectra is also attributed to the different excitation. In order to validate the color-tunable-emission of MYS: Ce3+, Mn2+ samples is similarly feasible in the CL aspect, we fixed the Ce3+ concentration at 1 mol % and changed the Mn2+ concentration from 5 to 18 mol %. Seen from Figure 9a, the CL intensity of Ce3+ gradually decreases 21889

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Figure 9. CL spectra of (a) MYS: x Ce3+, y Mn2+, (b) MYS: 1 Ce3+, z Tb3+, (c) MYS: 1 Ce3+, y Mn2+, z Tb3+ samples at different Ce3+, Mn2+ and Tb3+ concentrations: x = 0, 5 mol %; y = 0, 1, 2, ..., 18 mol %; z = 0, 1, 2, ..., 8 mol %. (e) The luminescent photographs of MYS: 1 Ce3+, y Mn2+, z Tb3+ (y, z mol %) samples: (1) y = 0, z = 0, (2) y = 2, z = 0, (3) y = 5, z = 0, (4) y = 10, z = 0, (5) y = 15, z = 0, (6) y = 18, z = 0, (7) y = 0, z = 1, (8) y = 0, z = 5, (9) y = 0, z = 8, (10) y = 3, z = 1, (11) y = 5, z = 1, (12) y = 5, z = 3, (13) y = 2, z = 1. (Accelerating voltage =3.0 kV, Filament current = 90 mA).

Table 1. CIE Chromaticity Coordinates (X, Y) of the MYS: Ce3+, Mn2+, Tb3+ under Low Voltage Electron Beam (Accelerating Voltage = 3.0 kV; Filament Current = 90 mA) point

product 3+

chromaticity coordinate (X, Y)

1

MYS: 1 Ce

(0.173, 0.087)

2

MYS: 1 Ce3+, 5 Mn2+

(0.265, 0.172)

3

MYS: 1 Ce3+, 10 Mn2+

(0.320, 0.221)

4

MYS: 1 Ce3+, 15 Mn2+

(0.381, 0.264)

5 6

MYS: 1 Ce3+, 18 Mn2+ MYS: 18 Mn2+

(0.471, 0.324) (0.489, 0.351)

7

MYS: 1 Ce3+, 1 Tb3+

(0.214, 0.231)

8

MYS: 1 Ce3+, 5 Tb3+

(0.245, 0.326)

9

MYS: 1 Ce3+, 8 Tb3+

(0.284, 0.437)

10

MYS: 1 Ce3+, 3 Mn2+, 1 Tb3+

(0.332, 0.337)

11

MYS: 1 Ce3+, 5 Mn2+, 1 Tb3+

(0.365, 0.333)

12

MYS: 1 Ce3+, 5 Mn2+, 3 Tb3+

(0.357, 0.295)

13

MYS: 1 Ce3+, 2 Mn2+, 1 Tb3+

(0.294, 0.275)

and the CL intensity of Mn2+ gradually increases with the increase of Mn2+ concentration, and its CL color is gradually transformed from blue to orange-red (Figure 9d, insets 1
6). The results also reveal that the efficient energy transfer occurs from Ce3+ to Mn2+ ions in the MYS host. Thus, the CL intensity of MYS: 1 Ce3+, 18 Mn2+ are much higher than that of MYS: 18 Mn2+ because of the Ce3+ f Mn2+ energy transfer, as shown in Figure 9a. For the MYS: Ce3+, Tb3+ samples such as MYS: 1 Ce3+, z Tb3+ (z = 1, 5, 8 mol %), their CL spectra (Figure 9b) consist of

the characteristic transitions of Ce3+ and Tb3+, and their CL colors can be changed from blue to green. Their CIE coordinates also confirm the above situation, which are (0.214, 0.231), (0.245, 0.326), (0.284, 0.437) for z = 1, 5, 8 mol %, respectively. Since MYS: Ce3+, Tb3+ can give a blue-green emission at a proper Tb3+ doping concentration and MYS: Ce3+, Mn2+ shows an orange-red emission due to the dual energy transfer of Ce3+ f Tb3+ and Ce3+ f Mn2+ when exciting under low-voltage electron beam, it is possible to obtain white light emission in the MYS host lattice by codoping with Ce3+, Tb3+, and Mn2+ and appropriately adjusting their concentrations. In our experiment, we have obtained excellent white light emission in the MYS: x Ce3+, y Mn2+, z Tb3+ systems. Figure 9c shows the CL spectra of the MYS: x Ce3+, y Mn2+, z Tb3+ (x = 1 mol %; y = 2, 3, 5 mol %; z = 1, 3 mol %) samples. Under low-voltage electron beam excitation, the MYS: x Ce3+, y Mn2+, z Tb3+ samples simultaneously show the characteristic emission of sensitizer ion (Ce3+, 5d1 f 4f1) and activator ions (Mn2+, 4T1 f 6S1; Tb3+, 5D3,4 f 7 FJ (J = 6
2). These emission lines of Ce3+, Tb3+, and Mn2+ cover the whole visible light region with tunable intensity, resulting in a tunable white light emission. The emissive color is bright white to the naked eye, as seen clearly from the luminescence photographs in the insets 10
13 of Figure 9d. The white emission can be further confirmed by the CIE coordinates of MYS: 1 Ce3+, 3 Mn2+, 1 Tb3+ (X = 0.332, Y = 0.337), MYS: 1 Ce3+, 5 Mn2+, 1 Tb3+ (X = 0.365, Y = 0.333), MYS: 1 Ce3+, 5 Mn2+, 3 Tb3+ (X = 0.357, Y = 0.295) and MYS: 1 Ce3+, 2 Mn2+, 1 Tb3+ (X = 0.294, Y = 0.275), respectively. In particular, the CIE chromaticity coordinate of MYS: 1 Ce3+, 3 Mn2+, 1 Tb3+ sample is very close 21890

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The Journal of Physical Chemistry C

Figure 10. Cathodoluminescence intensities of MYS: 1 Ce3+; MYS: 18 Mn2+; MYS: 1 Ce3+, 25 Mn2+; MYS: 1 Ce3+, 8 Tb3+; and MYS: 1 Ce3+, 3 Mn2+, 1 Tb3+ samples as a function of (a) accelerating voltage and (b) filament current.

to that of the standard equal energy white light illuminate (X = 0.333, Y = 0.333). These as-prepared MYS: Ce3+, Mn2+, Tb3+ phosphors with tunable white emission have potential as back light to applied in FEDs. The CL emission intensities of MYS: 1 Ce3+, MYS: 18 Mn2+, MYS: 1 Ce3+, 25 Mn2+, MYS: 1Ce3+, 8Tb3+, and MYS: 1 Ce3+, 3 Mn2+, 1 Tb3+ samples have been investigated as a function of the accelerating voltage and the filament current, as shown in Figure 10. When the filament current is fixed at 93 mA, the CL intensity increases with raising the accelerating voltage from 1.0 to 5.0 kV (Figure 10a). Similarly, under a 5.0 kV electron beam excitation, the CL intensity also increases with increasing the filament current from 80 to 92 mA (Figure 10b). There is no obvious saturation effect for the CL intensity of these samples with the increase of current density and accelerating voltage. The increase in CL brightness with an increase in electron energy and filament current is attributed to the deeper penetration of the electrons into the phosphor body and the larger electron-beam current density. The electron penetration depth can be estimated using the empirical formula:   n A E 1:2 pffiffiffi , n ¼ ð7Þ L½Å ¼ 250 F 1
0:29lgZ Z where A is the atomic or molecular weight of the material, F is the bulk density, Z is the atomic number or the number of electrons per molecule in the compounds, and E is the accelerating voltage (kV).42 For example, for MYS: 1 Ce3+, 3 Mn2+, 1 Tb3+, Z = 316.84, A = 680.75, F = 4.39 g/cm3, the estimated electron penetration depth at 3.0 kV is about 16.2 nm. For CL of the above samples, the Ce3+, Mn2+, and Tb3+ ions are excited by the plasma produced by the incident electrons. The deeper the electron penetration depth, the more plasma will be produced, which

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results in more activator ions being excited, and thus the CL intensity increases. The degradation property for phosphor is very important for FED application. Thus we also investigated the degradation behavior of MYS: Ce3+, Mn2+, Tb3+ samples under low voltage electron beam excitation. Supporting Information Figure S4a shows the decay behavior of the CL intensity of representative MYS: 1 Ce3+, 3 Mn2+, 1 Tb3+ sample under continuous electron beam bombardment when the accelerating voltage = 3.0 kV, filament current = 90 mA. The CL peaks are almost the same as those before electron bombardment. However, the CL intensity of the studied sample monotonously decreases with prolonging the electron bombardment time. After the continuous electron radiation for 1 and 2 h, the CL intensities of the MYS: 1 Ce3+, 3 Mn2+, 1 Tb3+ sample fall to 91% and 79% of the initial value, respectively. This degradation of CL intensity may be due to the accumulation of carbon at the surface during electron bombardment.43,44 The accretion of graphitic carbon during electron-beam exposure at high current densities is a well-known effect. This carbon contamination will prevent low-energy electrons from reaching the phosphor grains and also exacerbate surface charging, and thus lower the CL intensity. In addition, after stopping bombardment for a while the CL intensity could not restore to the initial value, indicating permanent damage to the phosphor occurs, which is another reason to result in the decrease of the CL intensity. On the other hand, the Commission International del’Eclairge (CIE) chromaticity coordinates of the MYS: 1 Ce3+, 3 Mn2+, 1 Tb3+ sample under a continuous electron beam radiation are measured to investigate the color stability and presented in Supporting Information Figure S4b (red line for x and black line for y). Obviously, the CIE values are nearly invariable under a continuous electron radiation for 2 h. x and y keeps at about 0.332 and 0.337, respectively, which corresponds to the white region in the CIE chromaticity diagram. In summary, the short time experiment (2 h) indicates that the stability of the CL intensity and CIE color coordinate of the asprepared MYS: Ce3+, Mn2+, Tb3+ samples is good, which shows potential advantages applied in the FED.

4. CONCLUSIONS In conclusion, Ce3+-, Mn2+-, and Tb3+-activated Mg2Y8(SiO4)6O2 oxyapatite structured phosphors have been prepared via high temperature solid state reaction. The Ce3+ ion simultaneously occupy the 4f and 6h sites in the MYS host and gives different blue emission under different UV excitation. There exists dual energy transfer (ET), that is, Ce3+ f Mn2+ and Ce3+ f Tb3+ in the MYS: Ce3+, Mn2+, Tb3+ systems under UV excitation. The energy transfer from Ce3+ to Mn2+ in MYS: Ce3+/Mn2+ phosphors has been demonstrated to be a resonant type via a dipole
quadrupole mechanism, and the critical distance (RC) calculated by quenching concentration method and spectral overlap method are 10.5 Å and 9.7 Å, respectively. By the dual ET of Ce3+ f Mn2+ and Ce3+ f Tb3+, the emitting colors of studied samples can be adjusted from blue to orange-red and from blue to green, respectively. More importantly, a wide-rangetunable white light emission from cool to warm white emission with high quantum yields (37
47%) were obtained in MYS: Ce3+, Mn2+, Tb3+ samples by precisely controlling the contents of Ce3+, Mn2+, and Tb3+ ions. In addition, a color-tunable emission in MYS: Ce3+, Mn2+, Tb3+ phosphors can be obtained by the modulation of excitation wavelength from 292 to 324 nm 21891

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The Journal of Physical Chemistry C due to the red shift of Ce3+ emission and the change of relative intensity of Ce3+ to Tb3+/Mn2+ emission. On the other hand, the tunable CL from blue to orange-red (Ce3+/Mn2+) and from blue to green (Ce3+/Tb3+), crossing the white region (Ce3+/ Mn2+/Tb3+) also be obtained via dual energy transfer. After the continuous electron radiation for 1 and 2 h, the CL intensity of the white-emitting MYS: Ce3+, Mn2+, Tb3+ sample remains 91% and 79% of the initial value, respectively, and its CIE coordinates are nearly invariable. The results indicate that the as-prepared MYS: Ce3+, Mn2+, Tb3+ phosphors have a good CL intensity and CIE coordinate stability under electron beam excitation. Generally, the white light with high quantuam (37
47%) and varied hues have been obtained in Ce3+-, Mn2+-, and Tb3+-activated MYS phosphors by utilizing the principle of energy transfer and properly designed activator contents as well as the select of excitation wavelength under UV (292
324 nm) and low-voltage (1
5 kV) electron beam excitation, which have potential as single-phase trichromatic white-emitting phosphors.

’ ASSOCIATED CONTENT

bS

Supporting Information. Structure parameters and ionic radii (Å) for given coordination number (CN) (Table S1); the XRD patterns of MYS host and different Ce3+‑ and Mn2+-doped MYS samples (Figure S1); cell parameters of MYS: Ce3+, Mn2+, Tb3+ samples at different Ce3+, Mn2+, Tb3+ concentrations (Table S2); dependence of PL spectra and emission (λex = 324 nm) intensity of Ce3+ and Mn2+ in the MYS: 1 Ce3+, y Mn2+ and MYS: x Ce3+, 5 Mn2+ samples on their concentrations (Figure S2); dependence of IS0/IS of Ce3+ on CCe+Mnn/3 (Figure S3); quantum yields and chromaticity coordinates of the MYS: Ce3+, Mn2+, Tb3+ under UV excitation (Table S3); and dependence of relative CL intensity and CIE coordinates of representative MYS: 1 Ce3+, 3 Mn2+, 1 Tb3+ sample on the radiation time (Figure S4). This information is available free of charge via the Internet at http://pubs.acs.org

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This project is financially supported by National Basic Research Program of China (Grants 2010CB327704) and the National Natural Science Foundation of China (Grants NSFC 60977013, 50872131, 20921002). ’ REFERENCES (1) Xie, R. J.; Hirosaki, N. Sci. Technol. Adv. Mater. 2007, 8, 588–600. (2) Lin, C. C.; Xiao, Z. R.; Guo, G. Y.; Chan, T. S.; Liu, R. S. J. Am. Chem. Soc. 2010, 132, 3020–3028. (3) Wang, J. W.; Tanner, P. A. J. Am. Chem. Soc. 2010, 132, 947–949. (4) Yang, J.; Zhang, C. M.; Peng, C.; Li, C. X.; Wang, L. L.; Chai, R. T.; Lin, J. Chem.—Eur. J. 2009, 15, 4649–4655. (5) Yang, W. J.; Luo, L. Y.; Chen, T. M.; Wang, N. S. Chem. Mater. 2005, 17, 3883–3888. (6) Won, Y. H.; Jang, H. S.; Im, W. B.; Lee, J. S.; Jeon, D. Y. Appl. Phys. Lett. 2006, 89, 231909. (7) Hsu, C. H.; Lu, C. H. J. Mater. Chem. 2011, 21, 2932–2939.

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